Transcriptional regulators such as nuclear receptors mediate distinct actions in different cellular and physiologic contexts by responding selectively to small molecule signals and by interacting differentially with an array of co-regulatory factors. The long term goal of this project is to understand the combinatorial principles that govern the interactions of nuclear receptors with cofactors, and the effects of ligands, other cofactors, and response elements on those interactions. The general strategy of the proposed research is to use a combination of molecular, genetic chemical and structural approaches to define pairwise and higher order interactions. The specific aims for this project are: (1) to define for various receptors (TR, GR, ER) the specificity determinants for interactions with LxxLL-containing peptide and coactivator fragments; (2) to identify TR LBD repression domains and test structural and functional relationships of activation and repression surfaces; and (3) to determine factor interactions at specific response elements in vivo and test interactions of functional surfaces in vitro. Nuclear receptors have been implicated in a wide range of diseases, including cancer, hypertension and inflammation, and IR ligands are widely used as therapeutics, diagnostics and chemopreventatives. Thus, understanding the principles and mechanisms of nuclear receptor action has important implications for health and for detecting, treating and curing disease.